U.S. patent number 5,749,044 [Application Number 08/502,793] was granted by the patent office on 1998-05-05 for centralized dynamic channel assignment controller and methods.
This patent grant is currently assigned to Motorola, Inc.. Invention is credited to Yih G. Jan, Kadathur S. Natarajan, Kenneth M. Peterson.
United States Patent |
5,749,044 |
Natarajan , et al. |
May 5, 1998 |
Centralized dynamic channel assignment controller and methods
Abstract
A central controller (40) executes a method (100) that selects
and assigns channels to serve mobile subscriber units (30) in a
space-based mobile telecommunication system (10). The method (100)
is based on simultaneous consideration of a number of criteria that
affect overall system performance of the mobile telecommunication
system. Another method (200) assigns serving cells (15-18) by
matching the available channel resources with actual caller demand
as a function of time. The methods (100, 200) may be used in
systems where the actual demand (i.e., offered subscriber traffic)
and the number of channels available in each cell (15-18) is
varying over time.
Inventors: |
Natarajan; Kadathur S. (Mesa,
AZ), Jan; Yih G. (Phoenix, AZ), Peterson; Kenneth M.
(Phoenix, AZ) |
Assignee: |
Motorola, Inc. (Schaumburg,
IL)
|
Family
ID: |
23999447 |
Appl.
No.: |
08/502,793 |
Filed: |
July 14, 1995 |
Current U.S.
Class: |
455/13.1;
455/12.1 |
Current CPC
Class: |
H04B
7/18539 (20130101); H04W 28/16 (20130101); H04W
84/06 (20130101) |
Current International
Class: |
H04B
7/185 (20060101); H04R 007/185 (); H04R 007/19 ();
H04R 007/195 () |
Field of
Search: |
;455/12.1,13.1,13.2,33.1,34.2,62,63,34.1,33.2,3.2,4.1,427,429,430 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Eisenzopf; Reinhard J.
Assistant Examiner: Arthur; Gertrude
Attorney, Agent or Firm: McGurk; Harold C.
Claims
What is claimed is:
1. A method for allocating channels and cells of a plurality of
satellites at each of a plurality of time epochs, the method
comprising the steps of:
a) calculating positions of each of the satellites at a time
epoch;
b) calculating positions of each of the cells for each satellite
based on the positions of the satellites at the time epoch;
c) calculating a carrier-to-interference value for each of the
cells at the time epoch based on a transmitting power of a cell and
interfering cells, antenna gain distribution of the cell and the
interfering cells, receiving antenna gain distribution of a
subscriber unit and receiver filter characteristics of the
subscriber unit;
d) allocating channels to each of the cells based on the
carrier-to-interference value at the time epoch; and
e) repeating steps (a)-(d) for each of the time epochs.
2. A method as recited in claim 1, wherein step (a) includes the
step of calculating the positions of the satellites by using an
orbit propagation model.
3. A method as recited in claim 1, wherein step (c) further
includes the step of calculating the carrier-to-interference value
additionally based on Doppler frequency shift and a time delay
offset between calls.
4. A ground station for allocating channels and cells of a
plurality of satellites at each of a plurality of time epochs, the
ground station comprising:
a central controller for calculating positions of each of the
satellites at a plurality of time epochs, for calculating positions
of each of the cells for each satellite based on the positions of
the satellites at each of the time epochs, for calculating a
carrier-to-interference value for each of the cells at each of the
time epochs and for allocating channels to each of the cells based
on the carrier-to-interference value at each of the time epochs
based on a transmitting power of a cell and interfering cells,
antenna gain distribution of the cell and the interfering cells,
receiving antenna gain distribution of a subscriber unit and
receiver filter characteristics of the subscriber unit; and
satellite communication means for receiving channel requests from
the satellites.
5. A method for dynamically selecting a candidate cell and a
corresponding channel to handle a channel request made by a
subscriber unit and received by a node, the method comprising the
steps of:
a) a central controller selecting the candidate cell having a best
overall weight of four criteria from a plurality of candidate
cells, the four criteria including broadcast power of each of the
candidate cells, carrier-to-interference value of each of the
candidate cells, channel availability of each of the candidate
cells and an estimated channel duration of a subscriber unit in
each of the candidate cells;
b) the central controller selecting a channel from a plurality of
channels in the candidate cell; and
c) the central controller notifying the node of the candidate cell
and the channel to service the channel request using the candidate
cell and the channel.
6. A method as recited in claim 5, further comprising the step of
the subscriber unit transmitting the channel request to the central
controller.
7. A method as recited in claim 6, further comprising the step of
the node receiving the channel request and relaying the channel
request to the central controller.
8. A method as recited in claim 5, further comprising the step of
the subscriber unit transmitting the channel request including
information about each candidate cell that can service the channel
request.
9. A method as recited in claim 8, wherein the step of transmitting
the channel request further includes the step of sending to the
node received broadcast power of each of the candidate cells, an
estimated cell duration time of each of the candidate cells and a
carrier-to-interference value of each of the candidate cells.
10. A method as recited in claim 5, further comprising the step of
the subscriber unit transmitting the channel request for handing
off communication from one channel to another channel.
11. A method as recited in claim 5, wherein step (a) includes the
step of selecting one of the candidate cells having a highest
ranking.
12. A method as recited in claim 11, wherein the selecting step
includes the steps of:
ranking each of the candidate cells in importance based on each of
the candidate cells' broadcast power, a carrier-to-interference
value, channel availability and estimated channel duration of a
subscriber unit; and
selecting the candidate cell having a higher degree of
importance.
13. A method as recited in claim 11, wherein the selecting step
includes the steps of:
ranking each of the candidate cells as primary, secondary and
tertiary factors based on each of the candidate cells' broadcast
power, a carrier-to-interference value, channel availability and
estimated channel duration of a subscriber unit; and
selecting the candidate cell based on a unique primary factor.
14. A method as recited in claim 13, wherein the selecting step
includes the step of choosing the candidate cell based on a unique
secondary factor if the primary factors are not unique.
15. A method as recited in claim 14, wherein the selecting step
includes the step of choosing the candidate cell based on a unique
tertiary factor if the secondary factors are not unique.
16. A method as recited in claim 5, wherein step (a) includes the
step of deciding which one of the candidate cells can best service
the channel request based on broadcast power of each of the
candidate cells, carrier-to-interference value of the candidate
cells, channel availability of each of the candidate cells and
estimated channel duration of a subscriber unit in each of the
candidate cells.
17. A method as recited in claim 5, wherein step (c) includes the
steps of:
the node receiving a message to service the channel request;
and
the node establishing communication with the subscriber unit using
the candidate cell and the channel.
18. A method as recited in claim 5, further comprising the step of
the central controller updating usage statistics for the node.
19. A method for dynamically selecting a candidate cell and a
corresponding channel to handle a channel request made by a
subscriber unit and received by a node, comprising the steps
of:
a) a central controller selecting the candidate cell from a
plurality of candidate cells;
b) the central controller selecting a channel from a plurality of
channels in the candidate cell;
c) the central controller notifying the node of the candidate cell
and the channel to service the channel request using the candidate
cell and the channel;
d) determining whether a time epoch has expired;
e) adjusting inter-cell relationships and usage statistics if the
time epoch has expired; and
f) repeating steps (a)-(e) once another channel request has been
received.
20. A ground station for assigning channels and cells of a
plurality of nodes at each of a plurality of time epochs, the
ground station comprising:
a central controller for receiving a channel request, for selecting
a candidate cell having a best overall weight of four criteria from
a plurality of candidate cells, the four criteria including
broadcast power of each of the candidate cells,
carrier-to-interference value of each of the candidate cells,
channel availability of each of the candidate cells and an
estimated channel duration of a subscriber unit in each of the
candidate cells, for selecting a channel from a plurality of
channels in the candidate cell and for notifying a node of the
candidate cell and the channel to service the channel request using
the candidate cell and the channel; and
satellite communication means for receiving the channel request
from the nodes.
Description
TECHNICAL FIELD
This invention relates generally to mobile telecommunication
systems and, in particular, to a central controller and methods for
performing dynamic selection and near real-time assignment of
channel resources to subscriber units in a space-based mobile
telecommunication system.
BACKGROUND OF THE INVENTION
Satellite cellular systems that assign channels to subscriber units
without considering the instantaneous load on the system are
limited from making the most effective use of local access
bandwidth. A certain amount of resources is pre-allocated for each
cell at each time interval based on expected traffic. However,
because of the stochastic nature of offered traffic, the static
approaches may lead to either resource wastage or shortage or
both.
In conventional static channel assignment methods, bandwidth
wastage occurs when actual channel or caller demand falls short of
the pre-allocated amount. Calls could be better allocated to
satellites which are experiencing less than expected caller demand
(i.e., under-utilized base stations). In the alternative, bandwidth
shortage may occur resulting in blocked and dropped calls when the
actual channel demand exceeds the pre-allocated amount. These
disadvantages, bandwidth wastage and shortage, are just a few of
the problems associated with conventional static channel allocation
methods when based on static or historic caller demands rather than
actual traffic demands. Accordingly, there is a significant need
for a dynamic channel allocation method and system that takes into
account the state of the telecommunication system when cell and
channels assignment decisions are made.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows in general a mobile telecommunication system;
FIG. 2 shows an example of four cells and a central controller;
FIG. 3 shows a flowchart of a method for allocating cells and
channels to subscriber units according to a preferred embodiment of
the present invention; and
FIG. 4 shows a flowchart of a method for dynamically selecting the
best cell and channel to handle a channel request according to a
preferred embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention describes a general method for assignment and
dynamic selection of channels to service mobile subscriber units in
a space-based telecommunication system. The present invention has
utility in enabling a telecommunication system to be more
responsive to actual traffic demand conditions by matching demand
with available channel resources as a function of time. The present
invention is "dynamic" because the state of the telecommunication
system is considered in deciding how channel resources are assigned
to meet the actual traffic demand. The invention is "centralized"
because all pertinent state information is resident in a central
controller which is responsible for making the channel assignment
decisions. State information may be computed at the central
controller, transmitted by the subscriber units to the central
controller or a combination thereof.
A "satellite" as used throughout this description means a man-made
object or vehicle intended to orbit the earth. A "satellite"
comprises geostationary, low-earth and medium-earth orbiting
satellites and/or combinations thereof. A "constellation" means a
number of satellites arranged in orbits for providing specified
coverage (e.g., radio communication, remote sensing, etc.) of a
portion, portions or all of earth. A constellation typically
includes multiple rings (or planes) of satellites and may have an
equal number of satellites in each plane, although this is not
essential.
FIG. 1 shows a general view of space-based telecommunication system
10 according to a preferred embodiment of the present invention.
Although FIG. 1 illustrates a highly simplified diagram of mobile
telecommunication system 10, system 10 comprises at least one
satellite 20, any number of subscriber units 30 and at least one
ground station 40 which includes a central controller. Generally,
communication system 10 may be viewed as a network of nodes. All
nodes of communication system 10 are or may be in data
communication with other nodes of communication system 10 through
communication links. In addition, all nodes of communication system
10 are or may be in data communication with other telephonic
devices dispersed throughout the world through public service
telephone networks (PSTNs) and/or conventional terrestrial
communication devices coupled to a PSTN through conventional
terrestrial base stations.
The present invention is applicable to mobile telecommunication
systems 10 having at least one satellite 20 in low-earth,
medium-earth or geosynchronous orbit. Satellite 20 is preferably a
satellite in low-earth orbit around earth. Satellite 20 may be a
single satellite or one of many satellites in a constellation of
satellites orbiting earth, like the IRIDIUM.RTM. system. Satellite
20 communicates with other nearby or adjacent satellites 20 through
cross-links. The present invention is also applicable to
telecommunication systems 10 having satellites 20 which orbit earth
at any angle of inclination including polar, equatorial or another
orbital pattern. The present invention is applicable to systems 10
where full coverage of the earth is not achieved (i.e., where there
are "holes" in the telecommunication coverage provided by the
constellation) and to systems 10 where plural coverage of portions
of the earth occur (i.e., more than one satellite is in view of a
particular point on earth's surface).
Each satellite 20 communicates with other nearby base stations
through a cross-link. These cross-links form a backbone of mobile
telecommunication system 10. Thus, a call or communication from one
subscriber unit located at any point on or near the surface of the
earth may be routed through a satellite or a constellation of
satellites to within range of substantially any other point on the
surface of the earth. A communication may be routed down to a
subscriber unit (which is receiving the call) on or near the
surface of the earth from another satellite 20. How satellite 20
physically communicates (e.g., spread spectrum technology) with
subscriber units 30 and ground station 40 is well known to those of
ordinary skill in the art.
Subscriber units 30 may be located anywhere on the surface of earth
or in the atmosphere above earth. Mobile telecommunication system
10 may accommodate any number of subscriber units 30. Subscriber
units 30 are preferably communication devices capable of receiving
voice and/or data from satellites 20 and/or ground station 40. By
way of example, subscriber units 30 may be hand-held, portable
telephone adapted to transmit to and receive transmissions from
satellites 20 and/or ground station 40.
How subscriber units 30 physically transmit voice and/or data to
and receive voice and/or data from satellites 20 is well known to
those of ordinary skill in the art. In the preferred embodiment of
the present invention, subscriber units 30 communicate with
satellite 20 using a limited portion of the electromagnetic
spectrum that is divided into numerous channels. The channels are
preferably combinations of L-Band and/or K-Band frequency channels
but may encompass Frequency Division Multiple Access (FDMA) and/or
Time Division Multiple Access (TDMA) and/or Code Division Multiple
Access (CDMA) communication or any combination thereof. Other
methods may be used as known to those of ordinary skill in the
art.
Ground station communicates with and controls satellite 20. There
may be multiple ground stations 40 located at different regions on
earth. For example, there may be one ground station 40 located in
Hawaii, another located in the Los Angeles area and another in the
Washington, D.C. area. Another example is to have separate ground
station 40 located in each country on earth.
Ground station 40 provides satellite control commands to satellite
20 so that it maintains its proper position in its orbit and
performs other house-keeping tasks. Ground station 40 is
additionally responsible for receiving voice and/or data from
satellite 20. How ground station 40 physically communicates (e.g.,
spread spectrum) with satellites 20 and/or subscriber units 30 is
well known to those of ordinary skill in the art.
FIG. 2 shows an example of four cells and a central controller. A
subscriber gains access to the network or system 10 via one of the
nodes 11, 12, 13, 14 (e.g., satellites). At any time instant, nodes
11-14 provide some type of radio frequency (RF) coverage,
represented as cells or zones 15-19, respectively. Nodes 11
provides RF coverage for cell 15, node 12 provides RF coverage for
cell 16, node 13 provides RF coverage for cell 17 and node 14
provides RF coverage for cell 18. Each cell 15-18 provides caller
access to subscribers within their coverage area. The actual number
of subscriber units that can be served simultaneously within cells
15-18 depends on the number of channels available. Nodes 11-14 may
have a fixed number of channels available (independent of time) or
a time-varying number of channels available.
For the purpose of providing complete coverage as well as increased
accessibility, nodes 11-14 may provide overlapping RF coverage
especially in geographic areas that have high demand for
connectivity. The overlapping coverage areas is shown in FIG. 2 as
that area where subscriber unit "M" is located. Those of ordinary
skill in the art will understand that cells or antenna patterns
generally represent regions where a signal level (for example, of
the broadcast channel) associated with a cell is greater than some
predetermined level, and outside that region, the signal level is
less than the predetermined level. As shown in FIG. 2, subscriber
unit "M" can be assigned a channel by nodes 12, 13 or 14. Although
the shape of the cells or zones shown in FIG. 2 is elliptical or
circular, the cell shape can be any shape for purposes of the
present invention. Moreover, although each of the nodes projects
one cell in FIG. 2, this invention is applicable to each of the
nodes projecting multiple cells.
The number of local access channels available for simultaneous
access by subscribers is limited. In order to handle a large number
of users using a limited number of channels, it is important to use
good channel management strategies and effectively utilize the
available channels. The subscriber units initially determine which
cell to communicate with based on the signal level or signal
quality of a channel received at the subscriber unit. For example,
a subscriber unit located within a center region of a cell or zone
would most likely choose to communicate within that cell because
the channel signal level of an antenna pattern is generally the
greatest in the center region. If a subscriber unit is located
within the region where two antenna patterns or cells overlap, the
subscriber unit may choose either cell to communicate with because
the channel signal levels are generally similar.
Central controller 40 shown in FIG. 2 is included in ground station
40 of FIG. 1. The central controller is responsible for first
allocating cells and channels during a first process and
dynamically selecting and assigning subscriber units to cells and
channels during subsequent processes (e.g., handoff). Central
controller may be for example, one or more computers which has
multiple processors and enough memory for performing the required
calculations and able to assign channels to potentially thousands
of subscriber units in a relatively short time period of time
(e.g., one second or less). The hardware of central controller 40
is well known to those of ordinary skill in the art. There may be
multiple central controllers 40, each located in a separate ground
station spread across earth.
FIG. 3 shows a flowchart of method 100 for allocating cells and
channels to subscribers according to a preferred embodiment of the
present invention. Method 100 is a software program that is
executed by central controller 40 located inside ground station 40.
Ground station 40 comprises the necessary computer hardware and
architecture for executing method 100. The computer hardware is
well known to those of ordinary skill in the art. Ground station 40
may execute other software programs for controlling the satellites
and other functions that are unimportant to the present
invention.
According to FIG. 3, central controller begins with a first time
epoch in step 102. There may be multiple time epochs, each time
epoch representing a time when the satellites and their
corresponding cells are at a particular orbital position. Each time
epoch may have the same or a different length of time. When central
controller dynamically selects and assigns cells and channels to
subscriber units requesting access to the telecommunication system
10 (FIG. 4), these time epochs are used to get the proper
orientation of telecommunication system 10 relative to earth.
Without knowing the current positions of satellites, central
controller would be unable to select and assign the best cell and
channel to service a channel request.
After step 102, central controller calculates in step 104 positions
of each satellite in the space-based mobile telecommunication
system using an orbit propagation model. An orbit propagation model
is a software program for determining positions of satellites as a
function of time as they orbit the earth. An orbit propagation
model is commercially available from the North American Air Defense
(NORAD) entitled Sattrack.
Central controller next calculates in step 106 positions of a
number of cells projected by each satellite at time T. This
calculation may be performed using the method described in U.S.
Pat. No. 5,227,802 to Miki Runnion and Ken Peterson entitled
"Description of Satellite System Cell Management".
Next, for each cell of each satellite, central controller
calculates in step 108 a received carrier-to-interference (C/I)
ratio. In a satellite telecommunication system with a number of
satellites and multiple cells per satellite, the interference
relationship of cells with respect to each other is time-varying
and depends on the position of the constellation at a given time.
Time is considered as a sequence of small intervals. Within each
interval, the inter-cell relationship stays fixed but may change
from one interval to the next. As time progresses, the trajectories
of the satellites are predictable and the locations of the cells
projected by a satellite can be predetermined in each time interval
(or epoch).
Central controller calculates the C/I ratio for each cell based on
the following factors: the transmitting power of the serving cell
and the interfering cell or cells, the antenna gain distribution of
the serving cell and the interfering cell or cells, the receiving
antenna gain distribution of the subscriber unit and the receiver
filter characteristics of the subscriber unit. Additionally, the
inter-modulation effects due to nonlinear amplifier
characteristics, the Doppler frequency shift and the time delay
offset between cells are also considered by the central controller
in the C/I determination. From the C/I values, a possible pool of
channel candidates is determined and assigned to this cell by
central controller in step 110. Possible channel candidates for
other cells can also be assigned. The assignment procedures may be
based on methods described in U.S. Pat. No. 5,268,694 to Yih Jan
and Ken Peterson, entitled "Communication System Employing Spectrum
Reuse on a Spherical Surface."
After step 110, central controller determines in step 112 whether
this was the last time epoch. If this is not the last time epoch,
method 100 advances to the next time epoch in step 114 and repeats
steps 104, 106, 108, 110 and 112 until the last epoch is reached.
Once the last epoch is reached, method 100 executed by central
controller ends.
FIG. 4 shows a flowchart of a method for dynamically selecting the
best cell and channel to handle a channel request according to a
preferred embodiment of the present invention. According to FIG. 4,
method 200 waits in step 202 until a subscriber unit transmits a
channel request for a channel to a satellite that is within
transmission range. The channel request comprises information about
each candidate cell that can service the subscriber unit, including
such information as the received broadcast power of each cell, the
estimated cell time and the carrier-to-interference value. The
channel request after arriving at the satellite is forwarded to the
central controller.
The channel request may be due to a handoff request which is
initiated by a subscriber unit. The handoff request may be one of
the following three types: an intra-cell handoff, an inter-cell
handoff and an inter-satellite handoff. An intra-cell handoff is a
handoff from one channel of one cell to another channel within the
same cell. An inter-cell handoff is a handoff from one channel of
one cell to another channel of another cell within the same
satellite. An inter-satellite handoff is a handoff from one channel
of one cell in a satellite to another channel in another cell of
another satellite. These types of handoffs are well known to those
of ordinary skill in the art.
The central controller maintains all relevant information of each
cell at each time epoch, including the following: (1) a cell's
broadcasting power as received by individual users; (2) a
calculated carrier-to-interference ratio or value; (3) an estimated
duration that a subscriber unit will remain in a particular cell,
(i.e., the estimated cell time); (4) a number of channels available
in each cell; and 5) a number of channels assigned to each
cell.
Based on the list of potential candidate cells that can service the
channel request, central controller determines in step 204 the best
cell for servicing the channel request. In the preferred
embodiment, the central controller determines the best cell based
on an evaluation of at least four criteria for each of the
candidate cells. The criteria are: (1) a candidate cell's broadcast
power received at the subscriber unit; (2) a candidate cell's
carrier-to-interference value; (3) a candidate cell's channel
availability in its channel pool; and (4) an estimated channel
duration of the subscriber unit in a candidate cell. In the
preferred embodiment, the four criteria listed above will be ranked
as primary, secondary and tertiary factors in order of decreasing
importance. The selection of the best cell to handle the channel
request will first be based on a primary factor. If the choice is
unique, the selection of the best cell is complete. If not, the
secondary factor is considered next and if needed, the tertiary
factors are considered until a candidate cell is determined to be
the best cell to service the channel request.
In an alternative embodiment of deciding which of the candidate
cells can best service the channel request, weighting factors may
be associated with each of the four criteria. A final selection can
be based on which cell has the maximum overall weight.
After the central controller finds the best cell, central
controller in step 206 selects a channel from the possible channel
pool of the best cell to service the channel request. The central
control notifies in step 208 the satellite having the selected cell
and channel to service the channel request. The satellite or node
commences to establish communication with the subscriber unit that
made the channel request using the selected channel and cell chosen
by the central controller. Communication is established using
technique well known to those of ordinary skill in the art.
Central controller next in step 210 updates the usage statistics of
the cell that was chosen by incrementing the "number of the
channels assigned" attribute associated with this particular
cell.
Central controller determines in step 212 whether the current time
epoch has expired. Every channel request occurs within a time
epoch. Time epochs are predetermined and may last any time
increment (e.g., 2 minutes, 5 minutes, or 10 minutes). Time epochs
are important because the carrier-to-interference ratio values of
each of the cells change as well as other inter-cell relationships
and usage statistics. If the time epoch has not expired, central
controller returns to step 202 to wait for a subscriber unit to
make a channel request. If the time epoch has expired in step 212,
central controller advances to the next time epoch in step 214 and
adjusts in step 216 the inter-cell relationships and the usage
statistics for each cell in the network. Central controller then
returns to step 202 to wait for another channel request. This
process continues indefinitely. Moreover, central controller is
able to execute method 200 in parallel for every channel request
received at the same time. In other words, central controller does
not have to wait until it determines what cell and channel to use
for one subscriber unit before selecting a cell and channel for
another subscriber unit or units.
During an initial channel assignment or during a handoff process
for a new channel allocation, the central controller will also
consider load balances among cells so that a channel assigned to a
subscriber unit may not come from a cell having the highest C/I
value. Whenever a call is terminated by a subscriber unit, the
central controller will free up the channel used by the subscriber
unit (since it is no longer needed) and update the cell's usage
statistics. Usage statistics of cells are updated in a similar
manner after the handoff events occur.
It will be appreciated by those skilled in the art that the present
invention dynamically selects and assigns cells to service mobile
subscriber units in a space-based mobile telecommunication system.
The dynamic channel assignment method performed by the central
controller enables the telecommunication system to handle
significantly more traffic than pure static channel assignment
schemes. With the disclosed method, the load distribution is
dynamically monitored by the central controller so that the traffic
load is evenly distributed in the telecommunication system. The
system capacity is effectively improved and the system performance
maintained at an acceptable level.
Accordingly, it is intended by the appended claims to cover all
modifications of the invention which fall within the true spirit
and scope of the invention, including an applicability of the
central controller and methods to terrestrial systems.
* * * * *